Fortran Hollerith strings are often cited as an example of a language which
is neither regular nor context-free.

Is there a formal classification of this type of language?

I have seen mention of "indexed languages" as a type of "context-sensitive"
language. does hollerith fall into this catagory? If so (or if not, even)
can anybody suggest a resource which covers "indexed languages" (or whatever
Hollerith is)